Sohail Akhtar is presenting on the topic of evaporation. The presentation will cover what evaporation is, the factors that affect evaporation, and methods to measure evaporation. Evaporation is when a liquid turns to a gas, such as water evaporating into water vapor. It is influenced by temperature, surface area, humidity, and wind speed. Evaporation can be measured directly using lysimeters, pans, or indirectly through the water budget, aerodynamic, or energy budget methods.

2. Student profile
Sohail Akhtar
20PWCIV5453
Class roll number: 175

3. Topic
Evaporation

4. Topics to be presented
Evaporation
Factors affecting evaporation
Measurement of evaporation

5. Introduction
Evaporation happens when a liquid substance becomes
a gas. When water is heated, it evaporates.
The molecules move and vibrate so quickly that they
escape into the atmosphere as molecules of water vapor.
Evaporation is a very important part of the water cycle.
Heat from the sun, or solar energy, powers
the evaporation process. It soaks up moisture from soil in
a garden, as well as the biggest oceans and lakes.
The water level will decrease as it is exposed to the heat
of the sun.

6. Process of Evaporation
Evaporation is a form of vaporization that usually happens on the surface of
liquids and it involves the transition of the liquid particles into the gaseous phase.
Therefore, this process is said to involve a change in the state of matter of liquids.
The surrounding gas must not be saturated with the substance which is
evaporating. They transfer energy to each other as the molecules of the liquid
collide, depending on how they collide with each other.

7. Continued….
The liquid particles will generally escape and enter the surrounding air as a gas
when a molecule near the surface consumes enough energy to overcome the
vapour pressure. The energy extracted from the vaporized liquid as evaporation
occurs will decrease the temperature of the liquid, resulting in a process known as
evaporative cooling.

8. Examples of Evaporation
 Drying of clothes in sun: We all dried up our clothes in the sun, the wet clothes get dries up
because of evaporation where the water turns into water vapor.
 Drying of water bodies: We have seen in summer water in the ponds and lake get decrease or
sometimes dried up due to evaporation.
 Water cycle: This is a very good example of evaporation, in the above two we discussed how
evaporation turns water into water vapor. This water vapor further goes to the sky and gets
condensed to form a cloud, and later it got precipitated. So, evaporation is significant to regulate
the water cycle.

10. Factors affecting Evaporation
Evaporation eases our life in many ways, and even it is contributing to
the water cycle too. But evaporation is dependent on some factors
which will be going to control the rate of evaporation. One more thing
to note evaporation is a slow process but some external factors
increase or slow down the rate of evaporation. Let’s discuss all these
factors of evaporation

11. Factors affecting Evaporation
The following are the main factors affecting evaporation
Temperature
Surface Area of
the Liquid
Wind Speed
Humidity

12. Temperature
The very first factor is temperature since we know evaporation can happen at any
temperature before the boiling point, but the temperature has a role in evaporation
too. It will determine the rate of evaporation, the more the temperature more is the
rate of evaporation. Now the question is how? We all know the temperature will
increase the kinetic energy, as the energy used to break intermolecular forces that
bind the liquid molecule. So, when we apply more temperatures the molecule
fastly breaks its intermolecular forces and get evaporated.

13. Surface area
Earlier we discussed that evaporation is a surface phenomenon, and the surface
plays a role in the rate of evaporation. The more a surface area more will be the
rate of evaporation. So, if more surface area will be there then more liquid
molecules will present on the surface, meaning more molecules break their
intermolecular bonds which will increase the rate of evaporation. So we can write
it as;
Surface area of liquid ∝ Evaporation

14. Humidity
Humidity refers to the amount of wetness or water vapor in the air. The more
water vapor in the air we will say more humidity. With more humidity, the rate of
evaporation gets decreased.
Humidity ∝ 1/Evaporation
For example, in the rainy season when our surrounding is more humid than it is
very difficult to dry up our clothes.

15. Wind speed
Wind speed is directly proportional to evaporation means with the increase in
wind speed there would be an increase in the rate of evaporation.
Wind Speed ∝ Evaporation
For example, on a windy day, clothes get quickly dried up than on normal days.
This is because the wind decreased the humidity which will increase the rate of
evaporation.

16. Measurement of Evaporation
It is measured as rate of evaporation. The rate of evaporation is defined as the
amount of water evaporated from a unit surface area per unit of time. It can be
expressed as the mass or volume of liquid water evaporated per area in unit of
time, usually as the equivalent depth of liquid water evaporated per unit of time
from the whole area. The unit of time is normally a day. The amount of
evaporation should be read in millimeters (WMO, 2003). Depending on the type
of instrument, the usual measuring accuracy is 0.1 to 0.01 mm.

17. Direct methods
Rate of evaporation
measurement
Indirect methods
Measurement of Evaporation

18. Direct method of evaporation measurement
The following are the main direct methods of evaporation measurement
Water budget Technique lysimeter Pan evaporation method

19. Water budget technique
This technique involves the maintenance of a water budget. Mathematically it is stated as
E = I - O ± ΔS
E = Evaporation I = inflow into soil reservoir
E= Irrigation water + precipitation + incoming soil water contribution from surrounding + G.W
O = deep percolation (inside soil runoff)
ΔS = Change in storage
Now I = i + P + SFI + SSQ + GW
O = RO + SSQ + DP + L + E

21. Lysimeter
A lysimeter is a measuring device which can be used to measure the amount of actual evapo-
transpiration which is released by plants, usually crops or trees. By recording the amount of
precipitation that an area receives and the amount lost through the soil, the amount of water lost to
evapotranspiration can be calculated. Monolithic weigh-able lysimeters are a tool for water balance
studies and solute transport determination. Lysimeters are of two types:
1. Weighing
2. Non-weighing

23. Pan Evaporation
1.Setup:
A standard pan, often painted white to reduce solar absorption, is filled with a known
amount of water. The pan is placed in an open, level area with minimal obstruction to air
circulation.
2. Measurement of Initial Water Level:
The initial water level in the pan is carefully noted before the start of the measurement
period. This serves as the reference point for calculating evaporation.
3. Exposure:
The pan is left exposed to the atmosphere for a specified period, usually 24 hours. During
this time, water evaporates from the pan's surface into the air.

24. Continue…
5. Correction Factors:
To account for factors that may affect the evaporation rate, correction factors are often applied.
Common corrections include adjusting for temperature, wind speed, humidity, and solar radiation.
These corrections help provide a more accurate estimate of evaporation.
6. Calculation of Evaporation:
The evaporation rate (E) is calculated using the formula: E=(Initial water level) -
(Final water level) + Precipitation - Runoff - Infiltration
7. Recording and Analysis:
The evaporation data, along with any correction factors, are recorded. Long-term records of pan
evaporation can be analyzed to identify trends, seasonal variations, and climatic influences on
evaporation in the specific location.

25. Pan Evaporation method

26. Indirect methods of evaporation measurement
Following are indirect methods of evaporation measurement
Aerodynamic
method
Energy budget
method
Hargreaves
method
Jensen Haise
method